Point f is on circle c. what is the length of line segment gf? 12.5 units 15.0 units 17.5 units 20.0 units

The
According to the image, the length of line segment GF can be found with
GF= GC +CF
CF is one the radius, so CF=7.5
to find GC, we can apply Pythagorean theorem on the right triangle GEC
it is 
GC² =GE² + EC² = 10² +7.5² =156.25, and then GC= sqrt(156.25)=12.5

therefore, GF= GC +CF=12.5+7.5=20

so the answer is 20.0 units